1. Field of the Invention
The present invention relates to a high luminance polarizing plate used in a display device and a method of manufacturing the same, and more particularly, to a high luminance polarizing plate having improved color, white luminance, and contrast ratio characteristics through stacking a λ/2 retardation film having an in-plane retardation value ranging from 250 nm to 280 nm in a wavelength range of 450 nm to 650 nm and a brightness enhancement film, and a method of manufacturing a high luminance polarizing plate, in which the polarizing plate may be manufactured using a roll-to-roll process.
2. Description of the Related Art
In general, a polarizing plate is composed of a structure in which a triacetyl cellulose (TAC) film as a protective film is adhered to a polarizer having a structure in which polyvinyl alcohol (hereinafter, referred to as “PVA”)-based molecular chains are aligned in a predetermined direction and an iodine-based compound or a dichroic polarizing material is included. The polarizer and the protective film are generally bonded with a water-based adhesive formed of a PVA-based aqueous solution.
In the case that a polarizing plate as described above is used in a liquid crystal display, a technique for improving brightness by attaching a brightness enhancement film, such as a Dual Brightness Enhancement Film (DBEF) by 3M, to one side of the polarizing plate has been suggested. However, in the case in which the brightness enhancement film is attached to a typical polarizing plate as described above, curling (hereinafter, referred to as “negative direction curling”) may occur, in which a plane at the center of the polarizing plate is downwardly convex due to the difference of shrinkage between polarizing plate and the brightness enhancement film under room temperature condition. In the case that negative direction curling occurs in the polarizing plate, corners of a liquid crystal panel are lifted due to the curling of the polarizing plate, and may come into contact with a case when the liquid crystal panel is mounted on a module of the liquid crystal display. As a result, the corners of the liquid crystal panel may be highly stressed, and thus, a light leakage phenomenon may occur therefrom. The light leakage phenomenon from corners of the liquid crystal panel may be the primary cause of a defective image.
Also, with respect to a typical polarizing plate having the brightness enhancement film attached thereto, brightness is improved. However, since black luminance as well as white luminance may be increased, a contrast ratio may be decreased, and bluish color may be present due to a color-shift phenomenon caused by the brightness enhancement film. Thus, display characteristics may be degraded in comparison to the case of using the polarizing plate alone.
With respect to the brightness enhancement film that currently is produced, such as DBEF, a transmission axis is formed in a drawing direction, i.e., a machine direction (MD). In contrast, with respect to a polarizing plate manufactured by drawing after dyeing a PVA film with iodine, an absorption axis is formed in a drawing direction. Therefore, since the transmission axis of the brightness enhancement film is generally formed in a direction perpendicular to a transmission axis of the polarizing plate based on the MD, a roll-to-sheet process, in which the brightness enhancement film is cut to correspond to a size and shape of the polarizing plate, and the cut film is then attached to the polarizing plate such that the transmission axis of the polarizing plate is matched with the transmission axis of the brightness enhancement film, has been used in order to attach the brightness enhancement film to the polarizing plate. However, in the case that the brightness enhancement film is cut to correspond to the size and shape of a sheet and attached to the polarizing plate, the failure rate may increase, as a separate cutting process may be required, the introduction of bubbles and foreign matter may be facilitated, and accurate alignment may be required. Also, in the case of using the roll-to-sheet process, productivity may decrease, because the number of manufacturing processes, along with the required manufacturing time, may be increased in comparison to a roll-to-roll process in which films are stacked by unwinding the films in the state of a wound roll and then cut. Furthermore, since parts of the polarizing plate having no brightness enhancement film attached thereto are not recycled and wasted, the roll-to-sheet process is disadvantageous in terms of product yield. In contrast, in the case of using the roll-to-roll process, since the brightness enhancement film is attached to the entire side of the polarizing plate, the brightness enhancement film may be cut according to the desired size of the polarizing plate, and other portions remaining after cutting the large-sized polarizing plate may also be recycled as a small and medium-sized polarizing plate.